1. Field of the Invention
The present invention generally relates to a semiconductor structure, in particular, to a semiconductor structure of a silicon controlled rectifier (SCR).
2. Description of Related Art
To preclude the internal circuit of an integrated circuit from being damaged by an electrostatic discharge (ESD) current, an ESD protection device capable of instantly transmitting a significant amount of ESD current is required to be arranged between a power rail and a signal pad. When an ESD positive pulse arrives at the signal pad, the ESD protection device is able to timely guide the ESD current of the signal pad to the power rail. By contrast, when an ESD negative pulse arrives at the signal pad, the ESD protection device is able to transmit the current from the power and/or ground rail to the signal pad. A silicon controlled rectifier is one of the most commonly used ESD protection devices.
The trigger voltage of the conventional silicon controlled rectifier is equivalent to a junction breakdown voltage of N-well and P-well. The junction breakdown voltage is significantly high and about more than ten voltages. In related art, N+ doped region is adopted to reduce the junction breakdown voltage to about ten voltages. However, the significant junction breakdown voltage is still high. In addition, the silicon controlled rectifier has parasitic capacitance. While the silicon controlled rectifier is applied to low-frequency circuits, the parasitic capacitance of the silicon controlled rectifier may be tolerated. However, while the silicon controlled rectifier is applied to the high-frequency circuits, the parasitic capacitance of the silicon controlled rectifier may pose an impact on the high-frequency signals of the signal pad and thus may affect the signal integrity.
Hence, how to reduce the trigger voltage and the capacitance of the parasitic capacitor of the silicon controlled rectifier without compromising the silicon controlled rectifier capability is one of the issues to be resolved in the pertinent field.